Gut Microbiome and Alzheimer’s – A Narrative Review

Document Type : Review Article

Authors

1 Microbiology, Faculty of Basic Sciences, Islamic Azad University, Mashhad Branch, Mashhad, Iran

2 Department of Laboratory Sciences, Kashmar Faculty of Medical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran

Abstract

Purpose:
Alzheimer’s disease (AD) is one of the most prevalent and complex forms of dementia, characterized by progressive decline in memory, cognitive abilities, and social functioning. With the global population aging, AD has become a major public health challenge. According to the World Health Organization (WHO, 2021), approximately 55 million individuals worldwide are affected by AD and other dementias, a figure projected to rise to 78 million by 2030 and 139 million by 2050. Recent research highlights the pivotal role of the gut microbiome in the prevention and management of AD. In this context, probiotics have emerged as a promising therapeutic approach, capable of modulating the gut microbiome to enhance brain health and mitigate disease outcomes. The present study aimed to evaluate the efficacy of probiotics and their potential mechanisms in alleviating Alzheimer’s-related complications.
Methods:
A systematic search was conducted in PubMed, Google Scholar, and ScienceDirect databases covering the period from 2003 to 2025. Inclusion criteria comprised clinical trials, animal studies, and narrative/systematic reviews directly addressing probiotics and AD. Exclusion criteria included studies lacking quantitative outcomes or unrelated to gut microbiome. The broad time frame was selected to capture two decades of evolving evidence in microbiome research.
Results:
Evidence from randomized controlled trials and meta-analyses indicates that probiotic supplementation can lead to measurable improvements in cognitive performance (average 8–15% increase in memory and attention scores), reduction in depressive symptoms (10–20% decrease in standardized scales such as GDS), and enhancement of overall quality of life indices in patients with AD. These effects are primarily mediated through the gut–brain axis, involving attenuation of immune-inflammatory responses, increased production of neurotransmitters such as serotonin and dopamine, and improved intestinal barrier integrity. Moreover, alterations in gut microbiota composition, particularly the enrichment of beneficial genera such as Lactobacillus and Bifidobacterium, play a crucial role in cognitive improvement.
Conclusion:
Probiotics exert beneficial effects in reducing anxiety and depressive symptoms and contribute to measurable improvements in cognitive function. However, current evidence is limited by small sample sizes, heterogeneity in probiotic strains and dosages, and short follow-up durations. Larger, long-term clinical trials and standardized protocols are required to validate these findings. Investigating the complex interactions between the gut microbiome and the central nervous system remains essential for designing complementary therapeutic strategies for neurodegenerative disorders, including AD. While promising, probiotic-based interventions should be considered adjunctive rather than standalone therapies.

Keywords

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Journal of Paramedical Sciences & Rehabilitation
Volume 14, Issue 4
February 2026
Pages 97-111

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